I still say, this is a result of collision damage which was subsequently repaired by a fly-by-nite body shop.
Notice the body parts illustration above showing part #'s 28212, 28300 & 28216. These parts are welded together, placed behind the door jamb where the latch is. They were factory resistance welded to the door jamb, floor and roof.
My guess is, when the body shop cut off the damaged fender, they probably cut off part of these structural braces or damaged them so they could not be welded back on to the replacement lower door jamb. Without the joining of the structure to the doorjamb, the assembly is greatly weakened. AND, if the door is not adjusted perfectly, it makes the problem that much worse.
JWA, I suggest taking apart the interior at that location and inspect for missing weld connections there.
Also, if the door does need adjustment, don't get it too tight to the body at the front as this will increase the stress at both the hinges and the latch mechanism. Just because the door leaks water doesn't mean the door is not adjusted right. Get better door gaskets (not the set you can get from Dennis Carpenter, etc).